This invention relates to a color image sensor and, more particularly, to a color image sensor for reading a color image out of a color document.
Color image sensors for reading color image of a color document adapted for facsimile systems have recently been attracting attention, and efforts have been made for their development. For example, Cd-Se color sensors are disclosed in Tajiri et al, "Study on color photoelectric circuits for proximity sensors" presented in the 1982th Conference of the Institute of Electronic and Communication Engineers of Japan. The sensors based on the Tajiri system are useful as color image sensors. The Cd-Se sensors, however, are limited in their speed of response to light. Also, their image reading rate is 20 msec. per line for monochromatic image at the most.
Further, in this system red, green and blue outputs of color image are obtained without use of any color filter but by switching light sources for the respective colors, i.e., LEDs (light-emitting diode) as red and green light sources and a fluorescent tube as blue light source, for each scanning line. Therefore, to obtain three color images three times the scanning period for monochromatic image is necessary, that is, a reading time of substantially 60 msec. per line is necessary.
Furthermore, photo-sensitive devices used for color sensors including the Cd-Se sensors noted above generally have peculiar spectrum distribution characteristics. Where color image signals from photosensitive elements of such a sensor are processed to re-construct image, therefore, the resultant displayed or printed-out image is liable to differ in color or brightness from the original image due to the peculiar spectrum distribution characteristics of the photosensitive elements. Heretofore, attention has not been paid to the peculiar spectrum distribution characteristics of the photo-sensitive devices and light sources used for color image sensors, nor has sufficient consideration been paid to the color and luminance of the reproduced image.